With growing consumer demands for energy, especially clean energy derived from renewable resources, photovoltaic devices (i.e., solar cells) have garnered increased interest and are becoming much more widely deployed in spite of their high cost. Silicon-based photovoltaic cells currently dominate the global photovoltaics market and are projected to continue to do so for the foreseeable future, with an estimated revenue growth of 10-25% taking place over the next 10 years. As a result of the expense of most photovoltaic cells, the photovoltaics industry is under tremendous pressure to cut costs, which can be a primary driver for a product's success in the marketplace. In many commercial applications, cost can outrank even a photovoltaic cell's performance and photon conversion efficiency.
Materials constitute a large portion of the total cost of silicon-based photovoltaic cells, and these costs are continuing to rise. The expense of high purity silicon is the leading cost driver for silicon-based photovoltaic cells. Alternative semiconductor materials are available, but they are also very expensive. The silver paste used to form electrical connections in conventional silicon-based photovoltaic cells ranks a close second behind silicon in terms being a cost driver. Silver prices have steadily increased and become quite volatile over the past several years, and there are limited opportunities to increase global silver production capacity. Despite these issues, there are currently no low cost alternative materials that can suitably replace silver in silicon-based photovoltaic cells, at least without necessitating significant changes to their manufacturing process.
During the fabrication of many conventional silicon-based photovoltaic cells, silver paste is applied to the top surface of the cell and converted into current collectors by a high temperature processing step (>800° C.). The high temperature processing step facilitates a glass frit etch of a SiN antireflective coating on the substrate, such that the silver can make electrical contact with the semiconductor. However, the micron-scale silver particles within the silver paste do not melt or become fused together with one another during the high temperature processing step. Instead, electrical conduction is established through grain-to-grain contact of the silver particles, thereby decreasing the obtainable electrical conductivity. The high temperature processing step also places significant restrictions on the types of materials that can be used prior to that operation. Thermally stable substrates capable of withstanding the processing temperatures of silver paste can also significantly add to the cost of current photovoltaic cells.
One of the most desirable features of silver in regard to the manufacturing of photovoltaic cells is its high electrical conductivity. Although other metals can display similar electrical properties, silver paste is still more readily processed than are any potential replacement bulk metals, thereby compensating for its high cost. Of potential metallic replacements for silver, copper presents particular advantages due to its similar electrical conductivity, much lower cost, and relatively low price volatility. By utilizing copper in place of silver in photovoltaic cells, material costs could be reduced by as much as 10%. However, copper presents particular challenges as a direct replacement for silver that have not allowed this change to be made.
Although photovoltaic cells having copper-based current collectors have the potential to fulfill an unmet need in the art, satisfactory means for fabricating such photovoltaic cells have yet to be developed. The present invention satisfies the foregoing need and provides related advantages as well.